UNDERWATER WINDMILLS


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Tidal Streams

Tidal stream turbines are
often described as underwater
windmills. They are driven by
the kinetic energy of moving
water in a similar way that
wind turbines use moving
air. The generator is placed
into a marine current that
typically results when water
being moved by tidal forces
comes up against, or moves
around, an obstacle or through
a constriction such as a passage
between two masses of land.
There are sufficient numbers
of such fast-flowing underwater
currents around the world
to make this form of marine
renewable energy worth pursuing.
In figure 1, the areas between the
coasts of Ireland and Scotland
that are coloured magenta would
merit the application of tidal
current capturing systems


Design challenges

There are three factors that
govern the energy capture by
any water current kinetic energy
converter: the swept area of the
rotor(s); the speed of the flow
(kinetic energy is proportional
to the velocity cubed) and
the overall efficiency of the
system. There have been many
challenges to make tidal turbines
commercially viable, among these
has been the need to place the
systems in the right locations
where the water depth, current
flow patterns and distance to the
grid make a project economically
viable, and to make units efficient
and easy to maintain.


Tidal power generation

As can be seen from the chart showing performance through
half a typical tidal cycle SeaGen generates according to its
design specification, namely 1.2 MW at 2.4 m/s. The red line
signifying the power output dips sharply at 11.15 am during a
demonstration of the potential for rapid power shut down on
command. The ramp up back to full power is slightly slower,
due to a grid constraint, but can be rapid, and the high degree
of controllability when under power is shown on the graph.
SeaGen produces fully grid-compliant electricity and the two
turbine rotors may even be operated independently with one
shut down; the system is in fact an ‘array’ of two independent
turbines on a single structure.


Future development

MCT is now concerned not only
with ensuring that its SeaGen
type device is installed in other
locations, but also with the
conception of new forms of
this technology that are both
more powerful (to gain further
economies of scale) and viable
in shallower and in deeper
water than the 20 m to 40 m
range that suits the current
design. In shallower water the
existing twin rotor system would
provide too small a swept rotor
area to be cost-effective, while
deeper water brings concerns
about taller tower structure
cost and strength.